Hypersilylphosphanylidene: A Facile Low‐Temperature Generation and Formation of Bis(hypersilyl)diphosphene and the Bis(hypersilyl)triphosphaallyl Anion

The reaction of hypersilyl(trimethylsilyl)chlorophosphane (SiMe 3 ) 3 SiP(SiMe 3 )Cl ( 1 ) with t BuOK in THF at –60 °C proceeds by Si–P bond cleavage of the trimethylsilyl group and formation of t BuOSiMe 3 /KCl and bis(hypersilyl)phosphanylidene. The phosphanylidene dimerizes to form bis(hypersilyl)diphosphene ( 2 ) in excellent yields (≥95%). X‐ray diffraction experiments reveal the simultaneous presence of two rotamers in the solid state that differ in their SiSiPP torsion angles. Ab initio calculations for the parent diphosphene (SiH 3 ) 3 SiP=PSi(SiH 3 ) 3 at the DFT/6‐31+G(d) level predict an energy difference of 2.3 kJ mol –1 . A cis arrangement of the two Si(SiH 3 ) 3 groups raises the energy by about 45 kJ mol –1 . In the UV/Vis spectrum the n → π* excitation is observed at 622 nm, which is in excellent agreement with the ab initio results. In the Raman spectrum, the P=P stretching vibration is easily identified as a strong line at 591 cm –1 . When a solution of 1 in THF is added to a solution of KO t Bu inTHF, maintaining an excess of KO t Bu during the reaction, the deep‐violet bis(hypersilyl)triphosphenide anion [HypPPPHyp] – ( 4 ) forms in excellent yields. This anion is also formed when 2 is treated with KO t Bu. Compound 2 can be derivatized with 2,3‐dimethylbutadiene to give a cyclic diphosphane 5 by [4+2] addition. When a solution of 2 in toluene is heated to 110 °C for 4 h, bis(hypersilyl)cyclotriphosphane (Hyp 2 P 3 H, 6 ) is formed, besides some HypPH 2 as a by‐product. As an aid for the interpretation of 31 P NMR spectra, equilibrium structures and phosphorus chemical shifts have been calculated at the DFT and MP2 level for a large number of molecules with phosphorus‐containing backbones, which are closely related to those described in the present work. Calculations for disilyldiphosphenes reveal a close correlation between PPSi bond angles and 31 P NMR chemical shifts, which is useful for predicting NMR spectra. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)